When a target-tracking radar, or a missile seeker assembly, incorporating electronically generated (that is, inertialess) nutation is operated in a countermeasures environment, difficulty is often experienced in maintaining lock-on. This is especially true when the target employs pulse jamming either at the nutation frequency or at a harmonic or sub-harmonic thereof. Such pulsing by the jammer in severe cases causes the radar to "break lock" on the target and in any event results in deterioration of radar performance, while a slight difference between the lobing frequency and the harmonic or sub-harmonic of the pulsing rate can cause the radar boresight to spiral around the target at this difference frequency.
An analysis of the effect of amplitude modulation on the radar return signal from a target shows that a first, or spurious, modulation may result from (1) a jammer of the barrage or repeater type, (2) from amplitude scintillation, or (3) from the amplitude component of jet engine modulation. Insofar as a pulsed jammer is concerned, practically any harmonic of the pulsing rate may contain sufficient modulation to cause difficulty. It is possible to utilize a balanced modulator to supply the radar nutation control signal and thus suppress modulation at the spurious frequency, but this expedient also suppresses the carrier. To recover the phase of the nutation (which is necessary to derive the antenna error signals) the carrier must be added to the suppressed carrier modulation; but, before re-insertion, the carrier must be limited to remove the spurious modulation. However, even with balanced modulation, spurious modulation at or near the nutation frequency is present if the spurious frequency is at or near twice the lobing frequency. Furthermore, the additional circuitry required to implement a balanced modulation network is not only costly but reduces overall system reliability.